A normal ear transmits sounds as shown in FIG. 1 through the outer ear 101 to the tympanic membrane (eardrum) 102, which moves the ossicles of the middle ear 103 (malleus, incus, and stapes) that vibrate the oval window and round window openings of the cochlea 104. The cochlea 104 is a long narrow duct wound spirally about its axis for approximately two and a half turns. It includes an upper channel known as the scala vestibuli and a lower channel known as the scala tympani, which are connected by the cochlear duct. The cochlea 104 forms an upright spiraling cone with a center called the modiolar where the spiral ganglion cells of the acoustic nerve 105 reside. In response to received sounds transmitted by the middle ear 103, the fluid-filled cochlea 104 functions as a transducer to generate electric pulses which are transmitted to the cochlear nerve 105, and ultimately to the brain.
Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea 104. To improve impaired hearing, auditory prostheses have been developed. For example, when the impairment is related to operation of the middle ear 103, a conventional hearing aid may be used to provide acoustic-mechanical stimulation to the auditory system in the form of amplified sound.
Middle ear implants also have been developed that employ electromagnetic transducers to mechanically stimulate the structures of the middle ear 103. A coil winding is held stationary by attachment to a non-vibrating structure within the middle ear 103 and a microphone signal current is delivered to the coil winding to generate an electromagnetic field. A magnet is attached to an ossicle within the middle ear 103 so that the magnetic field of the magnet interacts with the magnetic field of the coil. The magnet vibrates in response to the interaction of the magnetic fields, causing vibration of the bones of the middle ear 103. See U.S. Pat. No. 6,190,305, which is incorporated herein by reference.
Middle ear implants using electromagnetic transducers can present some problems. Many are installed using complex surgical procedures which present the usual risks associated with major surgery and which also require disarticulating (disconnecting) one or more of the bones of the middle ear 103. Disarticulation deprives the patient of any residual hearing he or she may have had prior to surgery, placing the patient in a worsened position if the implanted device is later found to be ineffective in improving the patient's hearing.
Novel surgical approaches try to deal with these issues by fixing an electromechanical transducer to the ossicle bones and placing an engagement member of the transducer against the oval or round window of the cochlear outer surface. The transducer is pressed toward the window membrane by filling fascia into the space between the transducer and a fixing anatomical structure. Fascia has the advantage of being biocompatible and having suitable damping properties to stabilize the fixing of the transducer and the engagement member to prevent their wandering out of place. But this approach very much depends on the exact execution of the filling of fascia and yields non-reproducible results. And either too much or too little exerted pressure on the membrane yields a distorted acoustic signal as perceived by the patient.